(1)Eighty-eight % of patients with interstitial pneumonia (IP) which complicated by collagen vascular diseases (CDV) showed that either surfactant protein (SP)-A or SP-D in sera was higher than their cut-off levels. In contrast, only 1.8% of patients without IP showed to be false positive. These results suggest the combination assay of SP-A and SP-D predominantly improves the serological diagnosis on IP complicated by CVD.(2)WE investigated the relationship between chest CT findings and SP-D/SP-A ratio in patients with idiopathic interstitial pneumonia (IIP). The patients were divided into two groups ; Group A showed predominantly an irregular linear opacity pattern on CT and histologicaly severe fibrosis with a weakly type II cell regeneration, Group B showed predominantly a ground-glass opacity pattern and histologicaly mild or moderate fibrosis with a prominently type II cell regeneration. As a result, Group A showed a significantly high mean value of SP-D/SP-A ratio when compared to that in group B.This suggests the heterogeneity in the rate of SP-A and SP-D production by two different cell types, type II cells and Clara cells.(3)We next investigated the relationship between serum SP-A levels and prognosis of patients with IIP.The IIP patients who died within 3 years showed significantly high SP-A concentrations when compard to the IIP patients who had been alive more than 3 years.(4)We also investigated the relationship wih IP comlicated by irradiation therapy. The patients came to show the increased concentrations of SP-A and SP-D in sera when IP was complicated by the received irradiation therapy. Therefore, a monitoring of the serum concentrations might be a useful tool to detect an onset of radiation pneumonitis. In the experiment using radiation-exposed rats, it was comfirmed that the incerase in sera consists with an appearance of pathohistological findings to be characteristic of IP.